1. Field of the Invention
The present invention relates to a noise-in-image-signal reducing circuit used in video disk players, video tape recorders, and television receivers.
2. Prior Art
Video signals consists of a series of frames which are used in video disk players, video tape recorders, and television receivers. Adjacent frames usually have a high correlation in terms of an image signal but have no correlation in terms of noise components contained in the adjacent frames. Thus, the correlation may be used to eliminate the noise components from the video images by averaging the images between frames with respect to time without any significant change in the signal energy of image.
FIG. 16 shows a prior art noise reducing circuit based on such a principle. In the figure, an input video signal A is delayed one frame by a frame memory 1 and is outputted as a delayed video signal B. A subtracter 2 subtracts the signal B from the signal A to produce the difference A-B in the density of picture element between the two signals A and B. The larger the motion is, the greater the value of A-B. The larger the motion is, the higher the frequency of motion is. A compensation signal calculator 3 receives the difference A-B and produces a compensation signal K(A-B) used for noise reduction where K is a multiplying factor. The compensation signal K(A-B) is then subtracted from the signal A to thereby reduce the noise components in the input video signal A. This type of noise reducing circuit is very effective for still pictures but is disadvantageous for moving pictures in that persistent images are developed in moving portions of an image resulting in blur in that image.
Thus, the value of K is varied in the range of 1.gtoreq.K.gtoreq.0 in accordance with the movements of image such that K is large to greatly reduce noise if movements are small, and K is small or zero to reduce noise for less blurred image if movements are rather large. FIG. 17 shows the relation of the difference signal (A-B) versus the compensation signal K(A-B). It should be noted that the K(A-B) is highest at a specific value the value of A-B, i.e., the magnitude of motion. It should be noted that the value of K(A-B) becomes zero for the value of difference signal (A-B) beyond a certain value i.e., when the motion in the image exceeds a predetermined level. This indicates that noise reduction is no longer effective beyond that value, preventing blurred images.
The difference signal (A-B) and corresponding compensation signal K(A-B) are tabulated in a certain relation and are previously stored in a memory such as ROM. Then, the compensation signal K(A-B) is outputted in accordance with the difference signal (A-B), so that the noise in an image is reduced in accordance with the motion of the image.
Experiments by the inventors of the present invention showed that the magnitude of motion in an image is not the only factor to cause a blurred image. The inventors discovered that a more blurred image could be developed for the same magnitude of motion when the frequency of motion falls in a particular frequency range. In order to obtain a high quality image, noise reduction should be controlled not only in accordance with the motion of image but also depending on the magnitude of the particular frequency components of the motion.
Further, as for color images, experiments by the inventors revealed that at high levels of color signal, color noise is more noticeable than blur of the image, and at low levels of color signal blurred image is more detectable than color noise. In order to obtain a high quality image, noise reduction should be controllably applied not only in accordance with the motion of image but also depending on the magnitude of the color signal components.